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256FERNANDO E. HIRATA ET ALLIThe study of Robertson & Mechoso (1998)also found a near-decadal cycle (approximately 9years) most marked in the streamflows of Paranáand Paraguay Rivers. The authors pointed out thatthis cycle is associated with sea surface temperature(SST) anomalies over the tropical North AtlanticOcean, mostly significant in austral summer. Anapparent co-variability of the 9-year cycle and SSTanomalies south of Greenland, also suggested arelationship of this cycle with the North AtlanticOscillation (NAO). Negative SST anomalies wouldbe associated with enhanced Paraná and ParaguayRivers streamflow. They also suggested a relation tothe decadal variability of the summer monsoonsystem and the southward moisture flux associatedwith the low-level jet along the eastern flank of theAndes Mountains. This impact would regulate arainfall see-saw between the region influenced bythe South Atlantic Convergence Zone and thesubtropical plains of SSA. The see-saw pattern isdescribed by Nogués-Paegle & Mo (1997).Numerical simulations presented by Robertson et al.(2000) showed that NAO interannual fluctuationsare highly correlated with SST anomalies in thetropical and subtropical South Atlantic Ocean,accompanied by variations in the summer monsoonover South America.Rainfall is the major source of long-termvariability in the hydrological cycle over SSA oncevariations in evaporation seem to be less important(Berbery & Barros 2002). Positive trends ofprecipitation were detected over the region andrelated to a change to more negative SouthernOscillation Index (SOI) conditions in the tropicalPacific Ocean since the 1960’s (Haylock et al.2006). Genta et al. (1998) focused attention on theexistence of long-term trends of streamflow in fourmajor rivers in the region (Uruguay, Negro, Paranáand Paraguay). They reported a general increase instreamflow beginning in the mid-1960s consistentwith a decrease in the amplitude of the seasonalcycle. In the case of the Negro River, the positivetrend began almost 15 years earlier, just after theextreme drought of 1943-45. Examining SSTanomalies in eastern equatorial Pacific Ocean, theysuggested that an important component of theobserved increase in streamflow is associated withlarge-scale low-frequency variability of the globalclimate system and the long-term trend is alsopossibly associated to changes in the Amazonregion.Long-term behavior of Pacific Ocean SSTconditions is usually associated with interdecadaloscillations (Mantua et al. 1997, Zhang et al. 1997).Abrupt changes from one phase of this interdecadalcycle to another are commonly referred to as regimeshifts. In 1977, the leading principal component ofSST anomalies of the North Pacific changed frommostly negative to mostly positive values (Mantua etal. 1997). Since then, there is uprising ambiguity inthe use of the term “regime shift” (Overland et al.2008). According to these authors, confusion arisesas a consequence of: (1) the shortness of climaticdatasets, (2) the lack of evidence on the existence ofdifferent stable modes of the climate system (eachstable mode would characterize one regime), and (3)the different usages of the term “regime” amidclimate scientists. Therefore, a clear definition of theterm is necessary.Here, we follow the “displacement”viewpoint of regime shifts and use the algorithmdesigned by Rodionov (2004) to identify them. Thedisplacement concept is solely based on time seriesanalysis of relatively short records (